Nowadays, technical procedures have widely used loads of equal or similar characteristics simultaneously. A very common example of this situation arises in study labs, classrooms or conference rooms having a considerable number of switching power supplies or chargers.
For instance, in the case of a massive application in study classrooms, where every student has a notebook, the simultaneous charge of such devices would be required. Taking into consideration that there may be as many as 40 students in the same class, 40 switching power supplies or DC chargers should be connected simultaneously when students leave school. With a 50 W average consumption for every source, we would get a 2,000 W consumption. With an alternating voltage supply of 220 VAC, it results in over 9 A of consumed current. While this value may not be high, the main problem arises when a multiple socket (a power strip), where all chargers are connected, is being plugged to the network. At this time, the inrush current highly exceeds nominal current.
Apart from the wear and tear impact and the useful life reduction of connection and wiring elements making up the power circuit, an additional effect may turn unfeasible the simultaneous connection. Quoting some values, the transitional phase of a switching DC charger may last around 300 ms. In the case of a single-phase power supply network rated 220 VAC, 50 Hz, protected with a thermo-magnetic switch of 20 A, Curve C, the instantaneous trip of such thermo-magnetic switch shall be between 5 and 10 times its nominal current value. Therefore, in the best-case scenario, when reaching 1=20 A×10=200 A, the switch shall open, to protect the installation. With an aggregate number of 40 DC chargers of 50 W each, a high inrush current would be obtained if they were all connected simultaneously, with a value even higher than 200 A. Therefore, the switch shall assume that is in presence of a “short-circuit” and shall open its poles, to protect the installation. The abovementioned situation has been detected, as mentioned, in classrooms. Clearly, the current state of technique does not contemplate these cases, as it did not deal with them. Therefore, at present, there are neither devices nor supplies that efficiently and comfortably solve this situation.
An instruction manual for the connection procedure may be considered but, besides the inconvenience, it should be taken into consideration that it shall be carried out by staff with a non-technical training, in many cases teachers/professors in areas completely separated from the technical field. Thus, no satisfactory results would be achieved. Moreover, the premature aging of connectors, cables, sockets, etc. has not been emphasized, but it should be considered.
Taking into consideration that switching chargers can keep constant their DC output voltage for a wide range of input voltages, in the case of 110 VAC supply situation, even with voltages lower than 100 VAC in many cases, for the same example above (40 sources of 50 W each), a nominal current consumption of 20 A would be obtained. At first sight, it may be inferred that the situation is not good either, in this case not because of inrush current issues, but because the steady state current shall be high. In the case of a circuit protected and calculated for nominal 20 A, 100% of consumption would be assigned to switching DC chargers only, with 0% available for other essential loads.